Fine structural studies on an ultraplanktonic species ofPyramimonas, P. virginica(Prasinophyceae), with a discussion of subgenera within the genusPyramimonas

Abstract

The ultrastructure of Pyramimonas virginica, the only known ultraplanktonic species of Pyramimonas, has been examined in detail. This species differs from other species of Pyramimonas in the structure of the flagellar apparatus, the scaly periplasL the pyrenoid, the eyespot, location of the eyespot in the cell, and mitosis. The flagellar apparatus is simple and shows signs of reduction. A 2-2-2-2 microtubular root system is present but a rhizoplast was not found. A very short transitional helix (perhaps two rings) is present in the flagellar transition region, together with a short one-piece stellate structure. Numbering of individual fagella in different species of Pyramimonas has shown a surprising variation in the orientation of the flagellar apparatus relative to the rest of the cell. Thus the eyespot in the various species is located differently relative to the flagellar apparatus and to other organelles such as the nucleus (in Pyramimonas there is no connection between the flage[lar apparatus and the eyespot via a flagellar root). Considerable experimentation has apparently taken place with regard to the orientation of the photoreceptor apparatus during the evolutionary history of the prasinophytes. In more advanced green algae this variability does not occur. In Pyramimonas four different flagellar apparatus/eyespot orientations have been detected, which has led us to a modified classification of the genus at the subgeneric level. New subgenera are proposed for P. virginica (subgenus Hexactis) and P. longicauda (subgenus Macrura).

Pyramimonas robusta sp.nov., a new punctate species, and a reappraisal of the subgenus Punctatae

“…Sym and R.N. Pienaar, in preparation;Hori et al 1995) and only seems to be constant in the universally accepted subgenus, Vestigifera McFadden (Hori et al 1995). No valid comment in this regard can be made on the other two newly erected subgenera, Macrura Hori, Moestrup, et Hoffman and Hexactis Hori, Moestrup, et Hoffman, as they are both monospecific (Hori et al 1995).…”

“…While, as a suite, these features are shared by all punctate species, few individually are unique to this subgenus (Table 2). Based on positional data of the eyespot apparatus and the nucleus relative to the basal apparatus, a strong alliance between the subgenera Punctatae and Trichocystis has already been shown, with the major discriminatory feature being the presence of trichocysts versus puncta (Hori et al 1995). However, a tree topology for the genus as a whole, derived from maximum likelihood analysis of rbcL gene sequencing, does not support a close relationship between Punctatae and Triclzocystis (Daugbjerg et al 1994).…”

“…14 and 16). However, those of different strains of P. olivacea have variable morphology (Hori et al 1995;@. Moestrup, personal communication), and there is some degree of overlap with the subgenus Trichocystis McFadden (Daugbjerg et al 1994).…”

“…In any event, the presence of this intervening structure in all our material of punctate species shows that the ultrastructure of the eyespot of P. mucifera is not unique. The apparatus in its entirety still remains somewhat enigmatic for the genus as a whole: it has up to four eyespots grouped in two closely appressed pairs in chloroplast lobes B and C and A and D (terminology of Hori et al 1995), the former situation being most like that of vestigiferans, Pyramimonas longicauda Van Meel (Hori et al 1995) and Pyramimonas chlorina sp.ined. (S.D.…”

“…1, 3, and 7) and orange-red. It is generally medial in situation along the length of the cell and in the right, ventral chloroplast lobe (lobe B sensu Hori et al 1995) near its right sinus and thus always associated with the nucleus (Figs. 15 and 16).…”